Examination of Paleogene distal humeri and a survey of homologous articulations in living primates allows some anatomical correlation with elbow mechanics and the occurrence of these during specific locomotor behaviors in living species. Claw climbing is postulated to be the ancestral primate locomotor mode from which the ancestral euprimate (strepsirhines and haplorhines) evolved a grasp leaping locomotor pattern. This pattern, which depends on a powerful grasp either during climbing or when landing after a jump, is still the most pervasive form of primate locomotion. Morphological evidence of Paleogene humeri suggest that vertical clinging and leaping behavior derived from grasp leaping at least six times independently.

Reconstructing the earliest phases of primate evolution has been impeded by gaps in the fossil record, so that disagreements persist regarding the palaeobiology and phylogenetic relationships of the earliest primates. Here we report the discovery of a nearly complete and partly articulated skeleton of a primitive haplorhine primate from the early Eocene of China, about 55 million years ago, the oldest fossil primate of this quality ever recovered. Coupled with detailed morphological examination using propagation phase contrast X-ray synchrotron microtomography, our phylogenetic analysis based on total available evidence indicates that this fossil is the most basal known member of the tarsiiform clade. In addition to providing further support for an early dichotomy between the strepsirrhine and haplorhine clades, this new primate further constrains the age of divergence between tarsiiforms and anthropoids. It also strengthens the hypothesis that the earliest primates were probably diurnal, arboreal and primarily insectivorous mammals the size of modern pygmy mouse lemurs.

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